Electrolytic reactor

ABSTRACT

An electrolytic reactor including a conical recess of removable slices through which the electrolyte circulates towards a part to be coated under the action of a pump setting up forced circulation. The part is polarized to act as a cathode, facing a coaxial anode in the recess. This configuration leads to good electrolyte flow in front of the part, allowing accelerated deposit with a uniform thickness of the coating material, and dimensions of the chamber may be added.

This presentation deals with an electrolytic reactor, used particularlyfor a surface coating application for a part used as an electrode.

Part coating by an electrolytic method is a well known technique thathas the advantage that it is inexpensive while in some cases it can beused to make deposits a few tens of microns thick, for example forcopper. Furthermore, it is easy to use this technique. Therefore it isused in preference to other techniques particularly for deposits made bysputtering or evaporation, in applications in which there is acompetition between the different techniques, such as for manufacturingparts in the micro-electronics and micro-mechanics fields.

However it is not always sure that the electrolytic coating will beformed satisfactorily. Defects that can occur include unequal thicknessover the surface or excessively slow growth of the electrolyticallydeposited material. Furthermore, providing an appropriate process, inother words for which the parameters give a satisfactory andreproducible result for the envisaged application, is not easy and theseparameters vary considerably with the application, which is particularlyundesirable for micro-systems which require highly dissimilar deposits.Finally, it is sometimes desirable to apply a magnetic field to thecoating to make a particular deposit with, a preferred magneticorientation of the material. This means placing a magnet around thecathode or around the reactor, and therefore limiting its size, and inpractice fixing the cathode to prevent alteration of the correspondingmagnetic characteristics of the material.

Thus, the following parameters have been found to be essential to obtaina uniform thickness deposit; the electrolyte, and particularly itsconductivity; the current density under continuous conditions orparameters of its pulses under pulsed conditions; the geometricarrangement of the reactor, and particularly its size and shape; and therelative positions and sizes of the electrodes; finally stirring andcirculation conditions of the electrolyte close to the part to becoated.

Displacement of chemical species in the electrolyte takes place bymigration which depends on the applied potential difference between theelectrodes, diffusion which depends on differences in the concentrationin the electrolyte, or convection which depends on stirring of the bath.But the predominant phenomenon at the location of the coating isdiffusion. Therefore, homogenous concentration of the material in theelectrolyte from which the coating is made is necessary in front of thesurface to be coated.

One known method of promoting circulation of the electrolyte and itsrenewal in front of the surface to be coated consists of moving a vanein front of the surface to be coated to stir the electrolyte. Anothermethod consists of circulating the electrolyte in a circuit using apump, this circuit passing in front of the surface. Theses methods areillustrated in U.S. Pat. No. 5,516,412.

These methods frequently give acceptable results, but they tend to causeturbulent stirring of the bath and therefore are not suitable for allsituations, and improvements are desirable.

The basic purpose of the invention is to make the electrolyte flowuniform, particularly in front of the part to be coated, and to make theelectric polarisation uniform to increase the uniformity of the coatingand its deposition rate.

Thus in its most general form, the invention relates to an electrolyticreactor characterised in that it comprises a conical chamber open at twoopposite ends, a support of a part to be coated (cathode) and acounter-electrode (anode) placed in the chamber or possibly at the exitfrom the chamber, towards the wide and the narrow end respectively, anda means of circulating the electrolyte through the chamber from thenarrow end to the wide end.

The progressive spreading of the flow towards the part to be coated,initiated well on the input side of this end, contributes to thispurpose.

The chamber is composed of stacked slices and a slice support andclamping armature such that the reactor has very attractive modularityproperties when it is applied to other parts with different dimensionsrequiring different geometric deposition parameters. The flow is moreregular in the conical chamber than at its exit, which is why it ispreferable to put the part to be coated in the conical chamber ratherthan at the said exit, even if acceptable results can still be obtainedat the exit. Slices are provided at the openings easily containing thepart to be coated and its support. This modularity is increased if atleast one of the slices contains a cavity in which the anode or itssupport can be placed, since it becomes possible to adjust the positionof the anode or to change its shape. It is preferable if a large numberof slices should have this property. Modularity can be used to adapt thedevice to a part to be coated with a given shape or surface area, or tomodify the distribution of current lines leading to the part (which iscalled the “diaphragm effect” described later). In the first case,slices 7 are added or removed on the upstream side of the part; and inthe second case they are added or removed on the downstream side of thepart.

It is recommended that the taper angle of the conical chamber should beuniform and less than 20°, while less than 14° is even better; theelectrolyte circulation should be coaxial with the conical chamberwithin a tank containing the said chamber, and the device shouldcomprise an electrolyte circuit looping back into the tank; and also theelectrolyte circuit should be connected to the narrow end of the chamberthrough a nozzle with a conical opening prolonging the chamber; allthese measures thus contribute to making the flow uniform.

Another improvement relates to the cathode (the part to be coated) andits support, since it is required that the means of fixing the part ontothe support should only slightly disturb the flow and should not formany major relief. The arrangement proposed for this purpose consists offixing the part with the same electrical contacts that polarise them;the support of the part to be coated then comprises electrical contactsfor cathode polarisation of the part arranged around the support andthat include a free end pressed in contact on the part, and a connectionend extending on a support face opposite the part. One ingeniousembodiment is characterised in that the connection ends of theelectrical contacts are connected to flexible arms of a star connector,fixed to the support by a mechanism with variable spacing, and in thatthe support includes stops on which the arms bend, and the electricalcontacts are in the form of curved hooks standing up on the arms.

The flow at this location is even better if the part to be coated andits support form a common smooth surface, namely if the part supportcomprises a housing with a periphery and depth adjusted to the part.

Finally, modularity is further improved if it also relates to thecathode holder, namely if the part support is installed removably on anarmature delimiting the conical chamber.

One essential feature of the invention is based on the fact that theconical chamber, the support of the part to be coated, the part itself,the anode and also preferably the arms of the circuit through whichelectrolyte is circulated through the chamber, are coaxial so as to moreeasily reach the target objectives; therefore the anode and cathodeelectrodes are suspended at the centre of the chamber.

The invention will now be described in more detail with reference to thefigures:

FIG. 1 shows a global view of the reactor;

FIG. 2 illustrates a slice delimiting the reactor chamber, and adjacentparts;

FIGS. 3 and 4 illustrate two states of the cathode holder;

FIG. 5 illustrates another slice;

FIG. 6 illustrates a detail of this slice;

FIG. 7 illustrates a complementary handling means;

FIG. 8 illustrates a disassembly slide; and

FIG. 9 illustrates use of this slide.

We will now consider the complete description of the invention withreference to the figures. The invention includes a tank 1 filled withelectrolyte and also containing a structure that forms the reactor 2itself, in other words the location at which the electrolysis takesplace and at which the coating is formed. A pump 3 circulates theelectrolyte through a pipe loop 4, the ends of which are connected ontoopposite orifices of the tank 1, creating circulation through thereactor 2. The tank 1 comprises stands 5 used to place it on a table oranother surface. The stands 5 are thus used to tilt the tank 1 toperform emptying or maintenance operations. The reactor 2 is composed ofa series of slices 7 stacked on each other or adjacent to each other,for which the outer edges are uniform. The slices 7 all comprise acentral conical recess, and these recesses prolong each other to form aglobal conical recess 13 (chamber) narrowing on one side at which thereactor 2 is adjacent to a side of the tank 1 and widening towards theopposite side of the tank 1, but without reaching it. This second side 8contains an orifice 9 through which the electrolyte is drawn into thepipe 4, while the side 10 described above is fitted with an injectionnozzle through which the electrolyte is sent into the reactor 2; thenozzle 11 also comprises a conical recess 12 that fits to the conicalrecess 13 of the reactor 2.

The slices 7 are approximately square, and are provided with a fewnotches like the slice shown in full in FIG. 2. One of these notches istriangular and is marked with reference 18 and the technician uses it toplace the slices 7 suitably in tank 1, by adjusting the notch 18 onto aslide 21 placed on the bottom of the tank 1. There are two other notches19 on the opposite sides of the slices 7 and are used to slide theslices on racks 22 fixed to the walls of the tank 1 and on which ablocking carriage 23 is made to slide compressing the stack of slices 7in the reactor 2. The slice 7 shown in the figure will be used to fix ananode 20 for which only the silhouette is shown in the figure and thatmay be a disk, a ring, a grid or any other structure depending on thedistributions of electrical current lines and electrolyte flow lines tobe set up. A solid anode 20 can reduce excess flow at the centre and anring-shaped anode 20 around the surface can concentrate the deposit ofmaterial in front of it, in other words close to the periphery of thepart to be coated. One interesting arrangement then consists of using aplurality of concentric anodes 20, extending at different radii andplaced on various slices 7 of the reactor 2, as illustrated by FIG. 1.If several anodes 20 are used at the same time, they could be polarisedindependently in order to apply different currents on each of them andthus compensate for any edge effects on the cathode.

Any bubbles present in the electrolyte (such as hydrogen generated bythe electrochemical reaction) can be captured with a diffusion grid 24without electrical properties placed in front of the cathode.

Regardless of the selected configuration, the anode 20 is housed in theslice 7 assigned to it by arms 25 that are inserted in vertical notches26 opposite the slice 7. The upper arm 25 contains an electricityconductor 61 and finishes on a connector 27 forced into a hollow 28 ofthe slice 7. The slice 7 also includes drillings 29 at mid-height in thehorizontal direction and into which pins 60 are fitted preventing theanode 20 from pivoting. The connector 27 holds a wire 61 leading to thepositive terminal of a dc current generator 62 illustrated in FIG. 1;the wire 61 is sheathed so that its length is immersed in the tank,except for the end that fits into the connector 27.

FIG. 3 shows details of the cathode holder 30 (part support). It isknown that the part itself for which the surface is to be coated acts asthe cathode in electrolytic coating processes. In this case, the part isa thin wafer 31 placed on a substrate 32 that comprises an anteriorhousing 33 with extent and depth adapted to the extent and depth of thewafer 31, such that it can fit with almost no clearance into it andwithout forming any projection or recess. This type of arrangementenables the electrolyte to flow in front of the cathode holder 30 andthe wafer 31. The substrate 32 also comprises a posterior housing 34with a circular step 35 in which a mechanical star 36 extends, formedfrom a central hub from which radiating arms 37 spread out and for whichthe ends bear on the step 35. The arms 37 support sheathed electricalcontacts 38 to provide electrical isolation from the electrolyte, andextend firstly obliquely through notches 39 formed around the peripheryof the substrate 32, and then forwards before curving in a half turn andfinishing at electrically insulating end pieces 40, preferably in theform of a suction cup so that the electrolyte current input onto thewafer 31 can be electrically isolated. Thus, the electrical contacts 38not only make the electrical connection with the wafer 31, but also makea mechanical attachment by holding it in the housing 33.

The star 36 carries a screw 41 that is held in it at a constant positionand for which rotation in a tapped part 42 of the posterior face of thesubstrate 32 causes the head to move upwards or downwards and thereforebends the star 36 by the arms 37 pressing on the step 35. This bendingis made possible by weak points 43 in the section of the arms 37 thatform hinge points. The arrangement is such that as shown in FIG. 4, thepenetration of the screw 41 and the bending of the arms 37 of the star36 cause the electrical contacts 38 to tip, which lifts the ends 40 ofthe wafer 31 and displaces them outwards, moving away from the wafer 31that can therefore be removed or replaced.

The number of electrical contacts 38 may vary if they are embedded inthe arms 37 by separable and particularly elastic connections. They mayalso be provided with a deformable button 44 pressed in throughdrillings of the arms 37 to be held in them at a constant position whilemaintaining electrical contact with the electrical wires 46 embedded inthe arms 37. The electrical wires 46 are connected through a conductingwheel 47 to a common connection wire 63 leading to the negative terminalof the generator 62. The modification to the number of electricalcontacts 38 also provides a means of adjusting the electrical currentcirculation and the electrolyte flow in and in front of the wafer 31.

The cathode holder 30 is held in an armature 64 by arms 65 similar tothe arms (25) in the anode 20.

Like the electrical wires 61 and 63, the electrical contacts 38 aresheathed where they are immersed in the electrolyte.

Due to the freedom of the arrangement made possible, particularly due tothe fact that the reactor 2 is divided into slices 7, the deviceproposed herein makes it possible to make fine adjustments to thehydrodynamic and electrical characteristics of the process and thereforeto more easily achieve a satisfactory coating on the wafer 31. It iseasy to modify the number and arrangement of electrodes, and it is alsopossible to modify the length and the section of the conical recess 13by choosing only some of the available slices 7. A more or lesspronounced “diaphragm” effect can be created at the cathode, by removingand adding the required number of slices 7. This effect is characterisedby the fact that current lines on the cathode edges can be limited andconcentrated in the central part.

It will be particularly pronounced if the ratio between the outputdiameter of the cone and the diameter of the substrate to be coated (thewafer 31) becomes smaller. This ratio can be varied by removing oradding slices 7.

Naturally and in most cases, the electrolytic deposition is made in theform of a dish with more material on the edges than at the centre.Creating a material defect on the edges makes it possible to smooth theprofile so that it tends towards a flat profile, therefore improving thehomogeneity of the deposit.

When the device has a given configuration for a given size of thesubstrate to be coated, the cathode holder 30 is adapted to treat asmaller substrate, and slices 7 can be removed until the desired effectis obtained:

-   -   diaphragm effect with a small taper angle;    -   normal flow with large taper angle.

Similarly, slices can be added to obtain a diaphragm effect on a largersubstrate.

The stack is compressed by the mobile carriage 23 free to move on theracks 22.

The small taper angle (about 2020 or less, and preferably about 14° orless) of the recess 13 can give excellent flow uniformity, which is evenbetter if geometric irregularities are small and particularly if thesurface of the recess 13 is very smooth; flow turbulence is then almostnon-existent.

It must be added that it is still possible to fix the wafer 31 bysuction on the cathode holder 30.

Finally, the magnetic polarisation magnets 66 in the wafer 31 can easilybe housed in the armature 64, in the cathode holder 30 or around thereactor 2 provided that they are placed such that they magneticallyorient the material deposited on the wafer 31.

The single nozzle 11 can be replaced by a stack of removable slices 7making up an adjustable nozzle.

The electrolyte injection velocity can be varied to modify the flowconditions and the electrolyte flow on the wafer 31.

Those skilled in the art will choose electrically insulating, chemicallyinert, hydrophilic materials with good mechanical strength for the tank1, reactor 2, etc.

An electrolyte retention tank may be placed on the pipe 4 on the outputside of the pump to adjust the electrolyte level in the tank 1,particularly when the reactor 2 is replaced by adding or removing wafers7, to empty the tank 1 or to fill it. Valves leading to the nozzle 11and to the retention tank are switched to allow the electrolyte to flowfreely into tank 1, or to discharge the electrolyte into the tank, or toenable normal flow in closed circuit inside pipe 4.

Some improvements to the previous embodiments are given in the finalfigures. FIG. 5 illustrates a slice 70 different from the previous slicedue to the presence of three notches 71 replacing the two notches 26 andnow placed at 120° from each other radiating inwards to receive an anodenot shown provided with three radiating arms 25 instead of two, as inthe previous embodiment. This device alone would prevent accidentalrotation of the anode, and better than would be possible with the pins60 that are not shown in this case.

In particular, the slice 70 is provided with grooves 72 at its vertex,and a portion 73 of the grooves 72 is free and another portion 74 isformed with a hook-shaped cavity, like that shown in FIG. 6 that shows across-section through this portion. The grooves 72 make it possible toinsert a handle 75 (FIG. 7) comprising a pair of vertical slices 76penetrating into the portions 73 and a pair of pins 77 aligned with eachother penetrating in the portions 74. When the handle 75 is engaged inthe grooves 72 and raised, it extracts the slice 70.

It has been observed that this extraction caused serious practicalproblems due to the propensity of the slices 7 or 70 to bond togetherwhen they are wet. With the device described above, the slices can beextracted more easily but it cannot be used in itself to separate them,such that several can be extracted at the same time. The device in FIG.8 is then used; it consists of a slide 78 provided with two flanges 79and 80, the second of which is separated by a slit 81. The flange 79 isprovided with a wheel 82 and the two halves of the flange 80 are eachfitted with a pin 83.

In this case (FIG. 9), the reactor comprises sidewalls 84 each with apair of grooves 85 and 86 close to their upper edges. When a slice 70has to be extracted, the top face of the tank is removed and the slides78 are installed on the sidewalls 84 such that their pins 83 enter thegroove 85 on the inside and the wheels 82 of the groove 86 on theoutside. It is then possible to move the slides 78 along the stack ofslices 70, stop them in front of the slice 70 to be extracted and retainthem by rotating the wheel 82 which makes it rub in contact with thewalls 84. The handle 75 then slides in the grooves 72 of the sliceconcerned, which is extracted by passing through the slits 81. Theslides 78 retain the adjacent slices.

1-14. (canceled)
 15. An electrolytic reactor, comprising: a conicalchamber open at two opposite ends; a support for a part to be coated; ananode placed in the chamber, towards a wide and narrow end respectively;and means for circulating the electrolyte through the chamber from thenarrow end to the wide end, wherein the chamber includes stacked andremovable slices and an armature for supporting and clamping the slices.16. An electrolytic reactor according to claim 14, wherein at least oneof the slices contains at least a cavity in which the support can beplaced.
 17. An electrolytic reactor according to claim 15, wherein ataper angle of the conical chamber is less than 20° and uniform.
 18. Anelectrolytic reactor according to claim 17, wherein circulating of theelectrolyte is coaxial with the conical chamber within a tank containingthe chamber, and comprising an electrolyte circuit looping back into thetank.
 19. An electrolytic reactor according to claim 18, wherein theelectrolyte circuit is connected to the narrow end of the chamberthrough a nozzle with a conical opening prolonging the chamber.
 20. Anelectrolytic reactor according to claim 15, wherein the support of thepart to be coated comprises electrical contacts for cathode polarizationof the part arranged around the support and that include a free endpressed in contact on the part, and a connection end extending on asupport face opposite the part.
 21. An electrolytic reactor according toclaim 20, wherein the connection ends of the electrical contacts areconnected to flexible arms of a star connector, fixed to the support bya mechanism with variable spacing, and wherein the support includesstops on which the flexible arms bend, and the electrical contacts arein a form of curved hooks standing up on the flexible arms.
 22. Anelectrolytic reactor according to claim 15, wherein the support of thepart comprises a housing with a periphery and depth adjusted to thepart.
 23. An electrolytic reactor according to claim 15, wherein thesupport of the part is installed removably on an armature delimiting theconical chamber.
 24. An electrolytic reactor according to claim 15,wherein the conical chamber, the support of the part to be coated, thepart itself, and the anode are coaxial.
 25. An electrolytic reactoraccording to claim 19, wherein the nozzle also includes stacked andremovable slices.
 26. An electrolytic reactor according to claim 15,wherein the slices are provided with individual extraction means.
 27. Anelectrolytic reactor according to claim 26, further comprising slidesfree to move in grooves of sidewalls of the tank and recessed above aslice to be extracted.
 28. An electrolytic reactor according to claim16, wherein the slice comprises at least three radiating anode supportarm cavities.